1 of 9) 1605928applications. [8][9][10][11][12][13][14] A spin quantum Hall state is also predicted in the distorted octahedral phase (1T′) of MX 2 in the monolayer limit, further extending applications of TMDs into spintronics and lowdissipation electronics. [13] As a part of the TMDs family, WTe 2 has recently attracted great interest due to its giant, nonsaturating magnetoresistance (MR) observed in bulk crystals, [15] and its predicted Weyl state. [16] Pressure-induced superconductivity and large spin-orbit coupling are also observed. [17,18] In addition, the lattice thermal conductivity of WTe 2 is predicted to be smaller than that of WSe 2 due to the heavier atom mass and the lower in-plane crystal symmetry. [19] Studies on WTe 2 have so far been carried out using bulk crystals or mechanically exfoliated flakes. Although mechanical exfoliation can produce high-quality flakes down to a monolayer, scaling it to obtain large-area thin films for practical applications is challenging. Thus, direct synthesis of WTe 2 thin films is desirable for potential electronic and thermal propertyrelated applications, but has yet to be realized due to the low bonding energy of W-Te. Synthesizing WTe 2 directly into largescale thin films is challenging due to its very small standard Gibbs free energy of reaction (−26.2 kJ mol −1 ) compared to that of WSe 2 (−135.0 kJ mol −1 ). [20,21] Additionally, the low melting point of the forming Te-W binary eutectic and high melting point of W (3422 °C) restrict the reaction efficiency between W and Te. Only recently, direct synthesis of MoTe 2 thin films, another interesting TMD [22] with a lower standard Gibbs free energy of reaction (−64.3 kJ mol −1 ) than WTe 2 , has been demonstrated via chemical vapor deposition synthesis (all values of standard Gibbs free energy of reaction are taken at 1100 K). [21,23,24] So far, no direct synthesis of large-area, highly crystalline WTe 2 thin films has been reported.Here, we demonstrate a large-area, facile synthesis of WTe 2 and MoTe 2 thin films by reacting sputtered metal films with H 2 Te, an intermediate vapor phase formed from Te vapor and H 2 carrier gas, through atmospheric pressure chemical vapor reaction. The synthesized films are polycrystalline whose grain size increases with increasing metal film thickness. Based on time-domain thermoreflectance (TDTR), [25,26] the in-plane thermal conductivity of our polycrystalline WTe 2 thin film is less than 2 W m −1 K −1 , at least 7.5 times smaller than that of single-crystalline exfoliated flakes (15 ± 3 W m −1 K −1 ) at room temperature. Through-plane thermal conductivity of our WTe 2 thin films was measured to be 0.8 W m −1 K −1 at room temperature, which is lower than that of the recently reported Large-scale, polycrystalline WTe 2 thin films are synthesized by atmospheric chemical vapor reaction of W metal films with Te vapor catalyzed by H 2 Te intermediates, paving a way to understanding the synthesis mechanism for low bonding energy tellurides and toward synthesis of single-crystallin...

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We report a scalable approach to synthesize a large-area (up to 4 mm) thin black phosphorus (BP) film on a flexible substrate. We first deposited a red phosphorus (RP) thin-film on a flexible polyester substrate, followed by its conversion to BP in a highpressure multi-anvil cell at room temperature. Raman spectroscopy and transmission electron microscopy measurements confirmed the formation of a nano-crystalline BP thin-film with a thickness of around 40 nm. Optical characterization indicates a bandgap of around 0.28 eV in the converted BP, similar to the bandgap measured in exfoliated thin-films. Thin-film BP transistors exhibit a field-effect mobility of around 0.5 cm 2 /Vs, which can probably be further enhanced by the optimization of the conversion process at elevated temperatures. Our work opens the avenue for the future demonstration of largescale, high quality thin-film black phosphorus.

Recent renewed interest in layered transition metal dichalcogenides stems from the exotic electronic phases predicted and observed in the single- and few-layer limit. Realizing these electronic phases requires preserving the desired transport properties down to a monolayer, which is challenging. Surface oxides are known to impart Fermi level pinning or degrade the mobility on a number of different systems, including transition metal dichalcogenides and black phosphorus. Semimetallic WTe exhibits large magnetoresistance due to electron-hole compensation; thus, Fermi level pinning in thin WTe flakes could break the electron-hole balance and suppress the large magnetoresistance. We show that WTe develops an ∼2 nm thick amorphous surface oxide, which shifts the Fermi level by ∼300 meV at the WTe surface. We also observe a dramatic suppression of the magnetoresistance for thin flakes. However, due to the semimetallic nature of WTe, the effects of Fermi level pinning are well screened and are not the dominant cause for the suppression of magnetoresistance, supported by fitting a two-band model to the transport data, which showed the electron and hole carrier densities are balanced down to ∼13 nm. However, the fitting shows a significant decrease of the mobilities of both electrons and holes. We attribute this to the disorder introduced by the amorphous surface oxide layer. Thus, the decrease of mobility is the dominant factor in the suppression of magnetoresistance for thin WTe flakes. Our study highlights the critical need to investigate often unanticipated and sometimes unavoidable extrinsic surface effects on the transport properties of layered dichalcogenides and other 2D materials.

The layered square-planar nickelates, Ndn+1NinO2n+2, are an appealing system to tune the electronic properties of square-planar nickelates via dimensionality; indeed, superconductivity was recently observed in Nd6Ni5O12 thin films. Here, we investigate the role of epitaxial strain in the competing requirements for the synthesis of the n = 3 Ruddlesden-Popper compound, Nd4Ni3O10, and subsequent reduction to the square-planar phase, Nd4Ni3O8. We synthesize our highest quality Nd4Ni3O10 films under compressive strain on LaAlO3 (001), while Nd4Ni3O10 on NdGaO3 (110) exhibits tensile strain-induced rock salt faults but retains bulk-like transport properties. A high density of extended defects forms in Nd4Ni3O10 on SrTiO3 (001). Films reduced on LaAlO3 become insulating and form compressive strain-induced c-axis canting defects, while Nd4Ni3O8 films on NdGaO3 are metallic. This work provides a pathway to the synthesis of Ndn+1NinO2n+2 thin films and sets limits on the ability to strain engineer these compounds via epitaxy.

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InxSn1−xTe is a time-reversal invariant candidate 3D topological superconductor derived from doping the topological crystalline insulator SnTe with indium. The ability to synthesize low-dimensional nanostructures of indium-doped SnTe is key for realizing the promise they hold in future spintronic and quantum information processing applications. But hitherto only bulk synthesized crystals and nanoplates have been used to study the superconducting properties. Here for the first time we synthesize InxSn1−xTe nanostructures including nanowires and nanoribbons, which show superconducting transitions. In some of the lower dimensional morphologies, we observe signs of more than one superconducting transition and the absence of complete superconductivity. We propose that material inhomogeneity, such as indium inhomogeneity and possible impurities from the metal catalyst, is amplified in the transport characteristics of the smaller nanostructures and is responsible for this mixed behavior. Our work represents the first demonstration of InxSn1−xTe nanowires with the onset of superconductivity, and points to the need for improving the material quality for future applications.

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